We propose to compare the structures of human and rodent (mouse and naked mole rat) amyloid-? (A?) fibrils using solid-state NMR. Transgenic mouse models are a mainstay of Alzheimer?s disease research. Many of these models include overexpression of A?, with various mixtures of human and endogenous mouse peptides in the different models. Although a great deal has been learned about the structure of human A? fibrils, much less is known about mouse A?, and species differences between rodent and human A? aggregates is underappreciated. This difference, however, could bear importantly on studies of neurotoxicity of A?. Some transgenic mouse models include massive overexpression of A?, and while this leads to some cognitive impairment, the burden of amyloid deposition seems disproportionately great compared to the neurological findings. There are several possible, non-mutually exclusive explanations for this discrepancy. Here, we propose to study one of them: that mouse and rodent A? aggregates differ structurally. It has been amply demonstrated that human A? peptides display polymorphism: that is, a single polypeptide can form different fibril structures depending on fibrillization conditions. These polymorphs differ in cyto-/neurotoxicity. Thus, this proposal is guided by the hypothesis that structural differences in mouse and human A? aggregates could lead to differences in neurological effects. As a first step in testing this hypothesis, we will compare human and rodent A?40 and A?42 fibrils. The proposal has two Specific Aims. The first is to compare the structures of A? fibrils from mouse, naked mole rat and human peptides using SS-NMR. These peptides differ at several key residues in the N- terminal domain of A?. Preliminary data (TEM, NMR) indicate that fibrils from these 3 species (rationale for these choices is given in the proposal), formed under rigorous controlled, identical conditions, adopt different structures. Aim 1 will continue these investigations. The second specific aim is to study and compare the structures of mouse and human brain-seeded A? fibrils. We have published several papers on producing ?replicate fibrils? for solid-state NMR, by seeding with authentic human brain amyloid from patients who died with Alzheimer's disease. We now propose to apply these methods to compare human and mouse brain amyloid-seeded A?40 and A?42 fibrils. Future plans are 1) to extend this work, in an R01 application, to a comparison of the cyto-/neurotoxicity of polymorphs directly; and 2) an application of these seeding techniques to examine brain amyloid-seeded soluble oligomers of A? peptides.